Change 'filename:' for Python 2 (legacy)

Before renaming, all Python 2 filenames were 'learnpython-*.py'.

This commit renames them to 'learnpythonlegacy-*.py'.

To verify that the filenames were named consistently across
translations prior to this commit, and to change this:

```
find . -name "pythonlegacy*.markdown" -exec ack filename: {} \;

find . -name "pythonlegacy*.markdown" -exec \
  sed -i 's/^filename: learnpython/filename: learnpythonlegacy/' {} \;
```

ko-kr/pythonlegacy-kr.html.markdown

@@ -3,7 +3,7 @@ language: Python 2 (legacy)
 category: language
 contributors:
     - ["Louie Dinh", "http://ldinh.ca"]
-filename: learnpython-ko.py
+filename: learnpythonlegacy-ko.py
 translators:
     - ["wikibook", "http://wikibook.co.kr"]
 lang: ko-kr